Measuring the Rastered Beam Spot Size on Target

Robert Michaels, rom@jlab.org, Jefferson Lab Hall A, March 2015

This file: hallaweb.jlab.org/equipment/daq/spot.html

SIMPLEST USAGE: Start a CODA run on L-arm (R-arm) DAQ and type "spot_L" ("spot_R") Note, the old notation "spot++" or "raster" or "raster++" are probably all synonymous with spot_L at the moment (I say probably because people tend to change these links). Run spot on aonl4 or similar computer on the a-onl account. Note: The beam must be on ( > 2 uA) for some time in the first 50000 events !!

MORE DETAILS ON USAGE:
- Type "spot_L" or "spot_R" with no arguments --> to analyze the most recent run.
- Type "spot_L 1062 0 200000" --> to analyze run 1062 (if it's still on adaq disks) from event 0 to event 200000.
- Type "spot help" --> to get help (note, there are other command line options, which "help" will explain).

If you ask for 2x2 mm raster, you probably will NOT get that. Why ? Because of quadrupoles between the raster and the target; note, the control software assumes a 23m drift with no magnetic fields. What to do ? With target out, ask for 2x2 mm (for example) and observe whatever you observe as the real spot size. Then multiply the requested size by the ratio of what you want to what you observe, and ask MCC for that. This should give you the desired spot size.
Do not confuse the "rastered spot size" (of order 4 mm) with the "intrinsic spot size" (typically 10 to 100 times smaller).
If your purpose is to check the spot size and dwell time to avoid destroying the target, then test at low current, say 1 microamp, before going to high current on target. Or test with no target in the beam.
A good double check is the oscilloscope trace of the raster current, seen in the middle room (electronic rack room) of the counting house.
Funny looking plot with a lot of scatter ? Possibly the beam was off, or sometimes off. Make sure the beam is on (> 2 uA).

More details about the raster are on the Raster section on Wiki